Use of 3-dimensional–Printed Guide in Hemisection and Autotransplantation of a Fusion Tooth: A Case Report Manuel Sato DDS , Alvaro Garcia-Sanchez DDS , Sergio Sanchez DDS and I-Ping Chen DDS, PhD Journal of Endodontics , 2021-03-01, Volume 47, Issue 3, Pages 526-531, Copyright © 2020 American Association of Endodontists Abstract Introduction 3D-printed guides have been used in endodontics to prepare conservative access, locate calcified or missing canals, and perform precisive osteotomy in apicoectomy. Here, we present treatment of a fusion tooth by combining 3D-printing technology and endodontic intervention in a 10-year-old patient. Case report The bifid crown of a maxillary right lateral incisor #7 had caused esthetic concerns and malocclusion. Clinical and radiographic examinations showed that #7 is fused with a supernumerary tooth with two independent root canals. The fusion involved the entire crown, coronal and middle roots. Due to financial constraints, a multidisciplinary approach involving endodontic, orthodontic and prosthodontic treatment was excluded. We hemisectioned the tooth intraorally with a 3D-printed guide, extracted the supernumerary tooth, and transplanted #7 to a position with improved esthetics and occlusion. A 3D-printed tooth replica was used to prepare the recipient site for autotransplantation. At 6-month follow-up, #7 was diagnosed with pulp necrosis and asymptomatic apical periodontitis. Root canal treatment of #7 was completed and osseous healing was observed 8 months later. Patient had no clinical symptoms and was satisfied with the outcome 14 months after hemisection and transplantation. The open space between #7 and #8 was closed without orthodontic treatment. Conclusions We present an alternative option to treat a fusion tooth in young patients who do not opt for other treatment options due to their stage of development or for socio-economic reasons. Techniques in modern endodontics, such as CBCT imaging and 3D-printing, should be adapted when it is beneficial to patients. Introduction Anomalies in tooth morphology, which can occur either in primary or permanent dentition, are rare but can lead to significant clinical problems in esthetics, occlusion, and function. A bifid crown is usually mesio-distally larger than a normal tooth and may result from fusion, concrescence or gemination. Fusion is defined as the union of dentin and/or enamel between two or more tooth germs ( ) . Fusion can be the union of teeth within the same dentition or a normal tooth with a supernumerary tooth. It is more frequent in primary dentition (0.5 to 2.5 %) than the permanent dentition (0.1 to 1 %) , . Concrescence occurs when two teeth are joined by cementum. The prevalence is 0.04 % and occurs most commonly in upper second and third molars , . Tooth gemination is when two tooth crowns arise from one tooth germ ( ) . Gemination is also more prevalent in primary dentition (0.5 %) than in permanent dentition (0.1 %) ( ) . To differentiate between fusion and gemination, the bifid crown is counted as one. Fusion is indicated when the tooth number is one less than normal. A complete tooth number indicates gemination. However, when fusion occurs between a tooth within the dentition and a supernumerary tooth, the number of teeth remains normal. A fusion tooth usually has two separate root canal systems whereas a gemination tooth has only one root and one canal. The etiology of fusion tooth remains unknown. The most common hypothesis is that physical force or pressure leads to direct contact of two tooth germs and causes necrosis of the epithelial tissue between two teeth during the developmental stage ( ) . Other possible causes include trauma, genetic and environmental factors , . The extent of fusion depends on the stage of tooth development at the time of union. Partial or incomplete fusion only involves the crown, while total or complete fusion involves fusion of both crowns and roots , . The root canal system of a fusion tooth can share a single pulp chamber dividing into two root canals or have two independent root canal systems , . Intervention of tooth fusion is needed when the tooth presents a pulpal/periapical/ periodontal infection or when esthetics and function are compromised. Several treatment options for fusion have been reported including extraction, root canal treatment, hemisection, and a multidisciplinary approach involving endodontic, orthodontic and prosthodontic treatment ( ) . To achieve appropriate treatment planning, it is essential to thoroughly analyze the root and canal anatomy of a fusion tooth. CBCT imaging offers images in all spatial planes and reconstructive volumetric images. It provides detailed information of root canal configurations, which may not be revealed by traditional radiographs ( ) . In addition, CBCT images merged with virtual images of teeth can be used to design a 3D-printed guide for non-surgical as well as surgical procedures , . In this case report, we describe successful treatment of a fusion tooth by hemisection and autotransplantation with the aid of a 3D-printed guide and a tooth replica. Case Report Clinical Examination and Treatment Planning A 10-year-old female patient with a bifid crown of #7 was referred to the Endodontic Clinic at University of Connecticut Health for evaluation and treatment. Medical history was unremarkable. Patient and her mother expressed that their main concern was esthetics and limited finances for dental treatment. The referring pediatric dentist further stated the concern of malocclusion caused by the heavy contact between the fusion tooth and mandibular right lateral incisor #26. Patient had no past history of pain. An extraoral examination showed normal tissues and no lymphadenopathy. The intraoral examination revealed a bifid crown of #7 with no caries or restoration ( Figure 1 A and B). Both crowns of the fusion tooth responded normally to cold (Endo Ice, Coltene, Alstätten, Switzerland) and EPT tests (SybronEndo Vitality Scanner, Kerr, Orange, CA). The fusion tooth had no mobility and all probing depths were less than 3 mm. A periapical radiograph showed a two-roots fused tooth with no apparent periapical pathology ( Figure 1 C). The CBCT image showed that the fusion involved the entire crown, coronal and middle roots ( Figure 1 D). Both roots of the fusion tooth had immature apices ( Figure 1 E). Based on the clinical and radiographic examinations, a fusion tooth of #7 and a supernumerary tooth with normal pulp and periapical status was diagnosed. To treat this fusion tooth with the least expense, we proposed to hemisection the fusion tooth, discarded the buccally-located tooth, and transplant the palatally-located #7 to achieve better esthetics and occlusion. We discussed the treatment plan and the guarded prognosis with the patient and her parents. We chose to discard the buccally-located tooth because the coronal pulp obliteration would make future root canal treatment more challenging ( Figure 1 F). The patient and her parents were advised that root canal treatment will be needed when the transplanted tooth becomes non-vital or symptomatic at follow-up appointments. Figure 1 Representative pre-operative intraoral photographs and radiographs of the fusion tooth: A) Buccal view; B) Occlusal view; C) Periapical radiograph; D) Coronal view showing the involvement of fusion; E) Coronal view showing the immature apex of the fusion tooth and; F) Axial view of CBCT images. White arrow indicates the coronal pulp obliteration of the buccally-located root. To avoid the tooth fracture during extraction and to minimize the extraoral operation time, we decided to perform intraoral hemisection. The CBCT image revealed that the crown of unerupted canine #6 was in close proximity to the cutting path of hemisection. We decided to fabricate a 3D-printed guide to separate two roots in a safe and controllable manner. Fabrication of the 3D-Printed Guide The procedures are described in 3 steps: 1. Image processing ( Figure 2 A and B): We took a digital impression of the upper arch with an intraoral scanner CEREC Omnicam (Dentsply Sirona, Bensheim, Germany). CBCT and intraoral images were processed as previously described ( ) . Briefly, CBCT images were segmented in an editor for DICOM files and all images were converted to STL files. An additional STL file of the palatally-located #7 was created for printing the tooth replica. These STL files were merged in a 3D triangular meshes editor software (Meshlab, ISTI, Pisa, Italy). After the merging process, the STL files were introduced into another 3D mesh editor software (Meshmixer, Autodesk Inc., Ontario, Canada). Figure 2 Design and fabrication of the 3D-printed guide: A) Merged image of intraoral scan and the STL file of CBCT image, showing the fusion of #7 and supernumerary tooth involving the crowns and 2/3 of the roots; B) Cutting path for hemisection as indicated by the pink line. The angulation was chosen to avoid damage to the permanent canine; C) Design of the 3D-printed guide with the proposed cut angle; D) Occlusal view of the designed guide. Arrow showing the cut window. The B-L dimension of the window was calculated to preserve buccal and lingual cortical bone; E) Final product of the 3D-printed guide. Arrows indicate windows of #A and #9; F) Extraoral fit of the 3D-printed guide. White arrow indicates the area where four openings on the side of the cutting window. 2. Design ( Figure 2 C and D): Design of the 3D-printed guide was based on the information revealed from the CBCT images, including the involvement of fusion, angulation and length of roots, the distance to adjacent teeth and the thickness of cortical bone. The cutting path was determined to be 1.9 mm away from tooth #6. The cutting window of the guide had a tight and precise fit of the shank of the high-speed bur. The bucco-lingual width of the window was designed to be 6.5 mm to achieve the complete separation of two roots and to preserve the buccal and lingual cortical bone. The 3D-printed guide spanned from tooth #A to #9 to ensure stability. Four openings were created near the cutting window to allow sufficient cooling with normal saline irrigation. 3. 3D-printing ( Figure 2 E and F): The guide and a tooth replica of #7 were printed in a 3D Printer (Formlabs 2, Formlabs, Somerville, MA) using FDA approved Dental SG Resin (Formlabs, Somerville, MA). Clinical Procedures We performed the clinical procedures under inhaled nitrous oxide and oxygen due to dental anxiety of the patient. Tooth #C to #8 were anesthetized by administering 2 carpules of 2% lidocaine with 1:100,000 epinephrine (Patterson Dental, Saint Paul, MN) . The fit of the guide was verified ( Figure 3 A). High-speed #862-012 flame burs with copious normal saline irrigation via the openings of the guide were used for sectioning the fusion teeth ( Figure 3 B). The depth of cut was 15.2 mm measured from the occlusal plane to achieve the complete separation of two roots. Figure 3 Clinical procedures of hemisection and autotransplantation: A) Intraoral fit of the 3D-printed guide; (B) Hemisection with the aid of 3D-printed guide; C.1) the buccally-located segment of the fusion tooth; C.2) the palatally-located segment of the fusion tooth; D) Preparation of the recipient site of autotransplantation with the tooth replica; E) Splint composed of fiberglass and flowable composite resin supporting the transplanted #7. We discarded the buccally-located crown and root of the fusion tooth ( Figure 3 C. 1 ). The palatally-located segment of the fusion tooth was extracted as atraumatic as possible and stored in saline solution ( Figure 3 C. 2 ). The socket site was prepared using the 3D-printed tooth replica ( Figure 3 D). #7 was then placed in the socket within 5 minutes and splinted from # 09 to # A with a fiberglass (Ribbond, Seattle, WA) and flowable composite resin (Filtek Supreme Ultra, 3M ESPE, Maplewood, MN) ( Figure 3 E). At the 2-week follow-up appointment, patient showed no pain to palpation nor percussion. Extraoral and intraoral examination revealed normal tissues. The splint was removed ( Figure 4 A). At the 3-month follow-up appointment, patient presented no clinical symptoms and the space between #7 and #8 was closed ( Figure 4 A). Mobility was improved from Miller class II at 2 weeks post-operatively to class I with normal probing depths (< 3 mm) except 4 mm at mesio-buccal site. The patient and her parents were advised that root canal treatment of #7 was likely needed because it did not respond to vitality tests and periapical rarefaction was observed. At the 6-month follow-up appointment, #7 still did not respond to vitality tests. #C had exfoliated and #6 was partially erupted. The periapical radiograph revealed enlargement of a radiolucent lesion around root apex of #7 ( Figure 4 B). Root canal treatment was completed in two visits. Access cavity was prepared under rubber dam isolation. The necrotic pulp tissue was extirpated, working length was determined with an apex locator (RootZX, Morita, Tokyo, Japan), the canal was prepared with 0.04 vortex files to final size 45 with copious 1% sodium hypochlorite (NaOCl) irrigation (Century Pharmaceuticals, Fishers, IN). Intracanal medication calcium hydroxide (Ca(OH) 2 , Pulpdent, Watertown, MA) was placed and tooth was temporarily restored with Cavit (3M, Saint Paul, MN) and FujiII (GC, Toky, Japan). Two weeks later, the canal was obturated with gutta-percha and AH Plus sealer (Dentsply Sirona, Bensheim, Germany) by the continuous wave technique. Radiographs taken at the 9- and 14-month follow-up appointments revealed partial and nearly complete osseous healing, respectively ( Figure 4 B). Over 14 months, the transplanted tooth exhibited class I mobility and normal periodontal probing. There was no occlusal contact between #7 and #26. Patient and her parents were satisfied with the treatment outcome. We will continue with annual follow-ups. As it is possible to adjust the position of an autotransplanted tooth by subsequent orthodontic treatment , , we will arrange an orthodontic consultation upon the request of patient and her parents as needed. Figure 4 Representative intraoral photographs and radiographs at 2 weeks, 3, 6, 9, and 14 months after hemisection and autotransplantation of the fusion tooth. A) Intraoral photos; B) periapical radiographs. wk: weeks, mon: months, F/U: follow up. Discussion The decision to perform hemisection intra- or extra-orally can vary from case to case depending on the extent of fusion and subsequent treatment procedures. Steinbock et al., report a 4-mm oval hole with pulp exposure that was created during the intraoral sectioning of a tooth fusion case ( ) . To completely separate this fusion tooth intraorally was challenging because the extent of fusion and the close proximity to the unerupted permanent canine. We thus designed and fabricated a 3D-printed guide to direct our sectioning path in a safe manner. Multiple applications of “guided endodontics” using 3D-printing technology have been reported. Here, we describe a novel application for 3D-printing in guiding the sectioning path of a tooth. It was relatively economical to fabricate this 3-D printed guide because we used a free and publicly accessible 3D mesh editor software. We also printed a tooth replica to prepare the socket site for the autotransplanted tooth. Using a tooth replica had been shown to increase the success rate of autotransplantation and survival of periodontal ligament cells of the original tooth, by eliminating repeated fitting of the donor tooth in the socket and decreasing the operation time , . Autotransplantation is a valid alternative when implants or prosthodontics cannot be performed in patients because of young age or for financial reasons. The overall survival rate of autotransplantation ranges between 60-98 %. Cohen et al., showed that the 10-year survival rate of transplanted anterior teeth with complete root formation ranges from 60-95% ( ) . Andreasen et al., found a 95-98% long-term survival rate for 370 transplanted premolars in a 13-years follow-up , . Factors associated with successful autotransplantation can be patient-, donor tooth-, recipient site-, and clinician-related ( ) . Better results are observed in patients who are younger, healthy, co-operative, and keep good oral hygiene. The favorable donor tooth is one that has intact periodontal ligament (PDL), minimal extraoral time, 3/4 to 4/5 root completed, the diameter of the apical foramina larger than 1 mm, and a single, cone-shaped root. Studies found that the viability of the PDL in extraoral environment decreases rapidly after 18 minutes ( ) . Lundberg and Isakasson showed 94% success rate of transplanted teeth with open and 84% success with closed apices over 5 years ( ) . Better healing can be expected in recipient site with adequate bone support and attached PDL tissues and clinicians who are experienced in atraumatic tooth extraction, preparation of the recipient site and tissue management. A non-rigid fixation for 7-14 days of the transplanted tooth has been recommended to promote bone healing and proliferation of PDL cells ( ) . Continuation of root development and pulp revascularization can be expected after transplantation with immature teeth ( ) . In this case report, the fusion tooth had an open apex thus root canal treatment was not performed prior to transplantation nor within 2 weeks after the procedure with the hope of regaining pulp vitality over time. The most common cause of failure following autotransplantation is root resorption, which is mainly due to injury to PDL cells. The most prevalent inflammatory root resorption often becomes evident within 3-4 weeks and replacement root resorption can be observed 3-4 months after the procedure , . Other consequences of autotransplantation may include marginal periodontitis, apical periodontitis, caries and trauma. To reduce the risk of PDL injury, we kept extraoral exposure time within 5 minutes and used a tooth replica for autotransplantation. Despite of these efforts, pulp vitality/healing did not occur and apical periodontitis developed after several months. This may be contributed by inadequate bone support and compromised periodontal tissues at the recipient site. The periapical lesion was resolved after root canal treatment. Conclusions In this case report, we implemented a 3D-printed guide for hemisection and a 3D-printed tooth replica for autotransplantation. 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